MacDon, the Assignee of the present application, manufactures a tractor which is used to transport headers of different types across the ground for cutting a standing crop.
Headers can be of various different types including headers known as “draper headers” in which there is provided a sickle knife across the front edge of the header which cuts the standing crop and a pair of conveyor belts or drapers behind the knife onto which the crop falls for transporting the crop transversely across the header to a discharge opening at a suitable location on the header. A reel is generally provided which rotates around an axis parallel to the knife above the knife and provides a series of bats which rotate and carry the cut crop over the knife and onto the conveyor belts.
Commonly at the discharge opening there is provided a crop conditioner into which the crop material from the discharge opening passes for conditioning of the crop to assist in drying.
It is necessary on a header of this type to drive the knife, the reel, the conveyor belts and the conditioner. Generally these elements each include a separate hydraulic drive motor.
On the tractor is provided a power source which drives generally two pumps which generate pressurized flow of hydraulic fluid which is fed to the motors to drive the motors at a required speed. The tractor includes control elements which allow control of the hydraulic fluid flow to control the speed of the elements as required under the actuation of the operator who has suitable control systems for controlling the speed of the various elements.
Tractors of this type are generally manufactured in a manner which will allow the header to be removed and replaced by an alternative form of header with the hydraulic connections from the control systems of the tractor to the various motors on the header including quick release couplings so that the header can be quickly disconnected mechanically and hydraulically.
Alternative forms of header which can be used on a tractor of this type include the auger header in which the conveying system of the drapers of the draper header are replaced by the conventional auger flight tube which is located behind the table and includes auger flight elements which carry the crop material inwardly toward a central discharge opening. In this header, therefore the auger must be driven a suitable drive motor in replacement for the drive of the drapers on the previous type of header.
Other types of header can also be used on a tractor of this type including a rotary cutter which includes a series of cutting discs at spaced positions across the width of the header with each disc carrying a pair of blades which rotate generally around a vertical axis of a disc. The discs thus carry the crop material which is cut inwardly to a discharge opening where the material exits into the conditioner. A header of this type requires therefore the driving of the discs and also the driving of the conditioner.
Hydraulic circuits for a tractor of this type therefore must accommodate the different types of header and provide the necessary motive power in the hydraulic fluid couplings to drive the motors of the header.
In recent years it has become common that the drive system for the header be arranged in such a way that the components can be driven in reverse in the event of a blockage or other problem which requires the crop material to be removed from the system. Thus it is necessary to drive the conditioner in reverse to drive any collected crop blockage back into the header. At the same time it is necessary to reverse the discs of a disc cutter and the auger of an auger header so that the crop material is carried back toward the front. It is common also to reverse the direction of the drive of the reel to again assist in moving the crop material to the front for discharge. The knife and the drapers can also be driven in reverse if this is convenient for the hydraulic circuit but of course reversing these elements has little effect on the operation.
The conventional technique for reversing the drive system in an arrangement of this type involves providing pumps which are bi-directional. Thus in a simple closed circuit, the pump of the tractor has its inlet and outlet directly connected to a corresponding port on the pump so that driving of the pump in the forward direction drives the motor in a forward direction and correspondingly driving of the pump in the reverse direction of fluid flow drives the motor in the reverse direction.
This arrangement is of course very simple but it has a number of disadvantages.
Firstly it requires the provision of bi-directional pumps which adds to the complexity of the system. It is difficult therefore in a manufacturing system to manufacture a tractor which has the option to omit the reversing system since all tractors will be manufactured with the bi-directional pumps regardless of the whether reversing option is selected or not.
Secondly, the bi-directional pumps require two directional flow in the circuit so that it is not possible to provide the conventional drain and filter system to reduce fluid contamination. This is particularly problematic in a system where quick couplers are used since contamination is more likely leading to possible damage to the system.
The problems with this type of drive for a windrower tractor that is designed to quick disconnect from the header implement, therefore, are:
The possibility of contamination where typically these types of machines run the oil in a closed loop with a makeup supercharge pump providing makeup (clean) oil. They also add shuttle valves to increase the makeup flow on the return side; however, typically this would only exchange 10% of the oil in order to have the contamination extracted from it. If the machine has quick couplers on it, chances of introducing contamination is high.
The requirement for positive shutoff where header shutoff on this type of machine needs to be positive to ensure there is no creep in the drive. This can be difficult to achieve consistently with a dual direction piston pump.
The limitations of reversing circuitry, since the dual direction pump is a closed loop pump, the reversing circuitry is limited to the drive system that it drives forward. This is typically not a problem on most auger header and disc mowers, as this is the normal drive system, but on draper headers and the auger header, auxiliary functions require variable speed controls which typically require a secondary pump to achieve. Reversing these components along with the main cutting/conditioning drive becomes a major challenge.
Reversing complexity is built into a standard drive arrangement rather than as an option. Dual direction pumps basically have all components necessary to reverse drives in the base design. Because of this, it builds more cost into the base unit which does not require or want drive reverse. On the standard draper header without conditioner drive reversing is not required.